1. Field of the Invention
The present invention relates generally to optical communication systems, and more particularly to an optical communication system that employs an optical transcription material in the receiver and that communicates through a turbulent medium.
2. Description of the Art
The telecommunications industry is rapidly switching to a hybrid platform which utilizes both electronics and photonics to increase the operational bandwidth. Today""s communication systems consist of optical fiber networks, fiber amplifiers, optical diode transmitters, and high speed semiconductor receivers. This architecture works well in the confines of optical fibers. There are problems in matching optical fiber network bandwidths, however, when propagating these signals in free space, which is necessary for remote applications.
Free space propagation of the signal through the atmosphere, water or other turbulent media will introduce fluctuating distortions and aberrations. These fluctuations prevent continuous focusing of the signal beam onto the small area high speed detectors typically utilized in optical communication systems.
A realistic example indicates that 0.5 m collection dish with a focal length of 1 m would concentrate the light into a diameter of 200 xcexcm for a 100 times diffraction limited beam at a wavelength of 1.5 xcexcm. This area is approximately 300 times larger than the high speed semiconductor photodiode detectors employed in communication systems. Aberrations similar to this could be incurred by atmospheric propagation. One approach is to correct for the distortions with adaptive optics, active tracking systems, or phase conjugation techniques in order to obtain a near diffraction limited signal beam which allows focusing onto the small area high speed detectors. These techniques suffer from slow response times, limited phase front correction or high signal intensities required for efficient conjugation. The other approach is to use a large area detector so that a significant fraction of the distorted signal beam can be collected by the receiver. This method has many advantages but has proven difficult to implement since the detector bandwidth (temporal response) and the detector area are often inherently coupled.
What is needed, therefore, is an optical communication system that is capable of communicating through a turbulent medium and which retains a large intrinsic bandwidth.
Transmission of an optical signal through a turbulent media, such as the atmosphere, produces a fluctuating spatial intensity pattern due to optical distortions and aberrations. With respect to FIGS. 1A, 1B, and 1C, three views are shown of an optical signal being transmitted through a turbulent media at three instants of time, t1, t2, and t3, respectively. These time varying distortions make it impossible to focus the signal beam onto a single small optical detector illustrated by the numeral 10 typically utilized in optical communication systems. The present invention involves collecting either a large enough subarray of the distorted signal (shown by the numeral 12) or the entire distorted signal (shown by the numeral 14 and encompassing the periphery in FIGS. 1A, 1B, and 1C) with an optical collector. The collected signal is invariant to the fluctuating distortions, thereby eliminating problems in free space propagation of optically transmitted high bandwidth signals.
The present invention involves combining an optical transcription material and an optical interconnect into an optical receiver while maintaining a fast temporal response, and thus a high bandwidth. This receiver directly measures temporal pulses, however, it is applicable to both temporally, phase, or frequency encoded signal sources. A separate transmission beam and probe beam architecture allows each of the optical wavelengths to be optimized for their own individual function. For example, the transmission wavelength could be chosen to increase signal throughput through a turbulent media while the probe beam could allow for dispersion free fiber propagation, or the fastest temporal response.
The optical transcription material (OTM) which will be described subsequently in more detail, utilizes a linear or nonlinear optical pump-probe mechanism to relay the information from the signal beam to the probe beam. The signal beam, also referred to as the pump beam, induces a time dependent index of refraction change, which is interrogated by a probe beam, also referred to as the reading beam. Through this mechanism, information which is encoded onto the signal beam is transcribed into amplitude, polarization rotation or phase modulation of the probe beam. The optical interconnect speed is limited by the intrinsic response time of the OTM as well as propagation delay time mismatches between the signal and the probe beams. These occur through two main sources, the delay time between the pump and the probe beams over the pumped volume (recognizing there is a 1 element delay) and the accumulated delays which occur over the entire optical interconnect.
Briefly, the present invention comprises an optical communication system for communicating through a turbulent medium. It includes an optical transmitter and an optical receiver. The optical receiver receives an optical signal containing information that fluctuates as it passes through a turbulent medium. It comprises a reflector for collecting the optical signal and for focusing it; a probe laser for generating an optical probe beam; an optical device having an OTM responsive to the focused optical signal and the probe beam and operative to change a characteristic of the probe beam, and optoelectronic detector means responsive to said changed characteristic and operative to develop an output electrical signal representative of the information contained in the received optical signal.
In another aspect, the present invention involves a time compensated probe methodology. Operational bandwidths in excess of 10 THz could ultimately be supported by this optical receiver methodology.
The foregoing and additional features and advantages of this invention will become apparent from the detailed description and accompanying drawing figures below. In the figures and the written description, numerals indicate the various elements of the invention, like numerals referring to like elements throughout both the drawing figures and the written description.